1. Technical Field
The invention relates to a structural element of high unidirectional rigidity, in which unidirectional fibre strands are completely embedded in sheaths and said sheaths are connected to a shell skin and wherein part of the shell skin is in the form of stringers whereby the shell skin has a skin plane and comprises a skin thickness. It also relates to a method of producing dimensioned amounts of fibre stiffness perpendicular to the skin plane of the shell skin in the case of large structural elements.
2. Background of Related Art and Summary of the Invention
Structural elements of high unidirectional rigidity are known. For example, EP 0 758 607 A2 and U.S. Pat. No. 5,735,486 disclose a wing having shear resistant wing shells made of fibre reinforced materials for use in aircraft. In the case of the wing, tensile and compressive force accommodating elements are disposed on the inner face of the wing shells. These are in the form of unidirectional fibers extending longitudinally of the wing. Mutually spaced stringers, whose fibre constituent is formed from a layered fibre structure which is connected to the layered fibre structure of the wing shell, are formed on the inner face of the wing shell longitudinally of the wing. Unidirectional fibre bundles, which are embedded in the shear resistant wing shells, are arranged between the mutually spaced stringers. They extend longitudinally of the wing and are substantially rectangular in section. The space between the stringers accommodates a plurality of fibre bundles and it is divided across its width by partition walls extending parallel to the stringers. The fibre portion of the stringers and/or of the partition walls is formed by folding at least the inner fibre layer of the torsion skin of the wing shell. The partition walls may be provided with layered sections which lie on the upper face of the fibre bundles.
The expression fibre structures is usually understood as meaning laminar structures which are formed by layering differently oriented unidirectional individual layers. A method of manufacturing conventional laminar structures is known from DE 37 39 753 A1. However, all of these structures have the disadvantages of the state of the art i.e. stresses perpendicular to the fibers, a danger that the laminations will part due to a lack of stiffness over the matrix layers, and restricted ability to dimension them due to the multi-axial stresses. In addition, it is not possible to provide a so-called fail-safe form of construction.
When using isotropic materials, an adequate degree of stiffness is needed in every direction subjected to loads. Consequently, it is known to use multi-layer laminates in order to obtain adequate degrees of stiffness in each direction. Since each of the direction related properties alters in dependence on the layer in accord with the orientation of the fibers, the tensile stresses in a cross-section of the wall are very inhomogeneous and the failure behavior is correspondingly complex. Thus, stress components may be at work which are effective in the direction of least rigidity. The rigidity of a purely longitudinally loaded unidirectional single layer is, for example, 50 to 250 times greater than in the case of transverse tensile loads. Fibre reinforcements having a unidirectional fibre orientation are, however, very much less sensitive to transverse compressive strains than to transverse tensile stresses, for which reason the rigidity is some three times greater in the case of transverse compressive loads. The transverse tensile stresses in particular can represent an unfavorable load on the boundary layer between the fibre bundles. Stresses are thereby conveyed in a direction perpendicular to the direction of orientation of the fibers. The resin layer between the fibre strands must be able to withstand these stresses. However, a corresponding problem is also presented by the stress in the boundary layer between the fibers and the resin of a transversely loaded unidirectional single layer.
Consequently, the object of the invention is to produce a structural element of the type mentioned in the first part of claim 1 which overcomes the aforementioned problems and in which considerably higher failure limits can be achieved by constructional measures than are attainable when using currently available laminar structures.
In the case of a structural element of the type mentioned in the first part of claim 1, this object is achieved in that there is provided an inner division of the cross-section of the shell skin thereby providing the strength of the fibers and the stiffness of the fibers in a direction perpendicular to the skin plane for every direction of loading, and in that one or more parting layers and/or a joint are provided in or on an upstanding skin fold and/or within the shear resistant sheath of a fibre strand.
The invention is based on the realization that the requisite rigidity cannot be matched to the flow of forces in the structure by providing a large area textile structure consisting of differently oriented individual layers. Rather more, a partially isotropic structure would thereby be produced which, additionally, has a low degree of stiffness perpendicular to the plane of the shell skin and is inclined to become delaminated.
The special feature of the solution in accordance with the invention lies in the deliberate usage of the decoupling caused by the envisaged parting layer for the anisotropic structural design. This decoupling results in directed frictional connections in each of the spatially separated regions which can be optimally withstood by parallel disposed fibers, i.e. which can be overcome. The stress, the stiffness and the strength are all oriented in the same direction.
The crucial point of the advantage of this solution lies in the freedom obtained in this manner of being able to design an entire structure in such a manner that the course of the frictional connections is optimal in the sense of the behavior of the structure. Unfavorable loading of the fibers is avoided to the widest possible extent. In addition, premature failure is practically eliminated.
Good utilization of the high rigidity of the fibers is made possible by virtue of the principle underlying the invention. The failure behavior of the structure is determined by the fibre stiffness and no longer, as was frequently the case in the state of the art, by the secondary stiffness transverse to the orientation of the fibers. As a result of the invention, an anisotropic design is now possible for the entire structure instead of the isotropic designs using anisotropic materials that are conventional today.
It has already been proposed in DE 197 30 381 C1 and GB 2 331 282 A, to additionally equip structural elements of high unidirectional rigidity with parting layers which interrupt the stress flow between adjacent unidirectional fibre bundles. This step is very advantageous, but as a sole measure, the result thereof is that all of the force flows between the unidirectional strand elements are interrupted except in the case of a compressive loading when the boundary faces are contacted. Due to the invention however, there is, in addition, a complete decoupling action, even under compressive loads, in the sense of an optimal loading of the fibre strands.
By using the measures in accordance with the invention, the advantage is obtained that one can avoid the failure of a component, as occurs when using unidirectional individual layers in a multi-layer composite, wherein stress components transverse to the orientation of the fibre occur under load, and shear stresses are generated which can cause the undesired failure of the component at values far below the theoretical values or those measured in the fibre strands. Through the formation of a structure in accordance with the invention, a structural element is obtained in which appreciably higher failure limits are produced since the critical stress components are, to a large extent, avoided through the construction of the structural element. In the structural elements designed in accordance with the invention, the inner force flows are forced towards the load bearing fibers or fibre strands. The rigidity of the fibers can be fully utilized due to the concurrence of the force flow direction and the orientation of the fibers since the fibers are merely subjected to a purely longitudinal loading force. The structural element is advantageously loaded in accord with the fibre properties.
Consequently, it is particularly advantageous if the fibre strands are substantially fully decoupled from transverse and shear loads. However, in order to obtain a simpler solution insofar as the construction thereof is concerned, superimposition of the less critical transverse compressive strains can be accepted. It is then advantageous if the compressively loaded fibre strands are supported against buckling, As a result of the decoupling, force flows cannot be conveyed via the sheaths surrounding the fibre strands. The sheaths thereby enclose the fibre strands like cells. As a result, the enclosed fibre strand behaves like a very longitudinally stiff yet pliable rod which cannot accommodate transverse load components without external support nor compressive forces without sufficient support against buckling. In accordance with the invention, this support is provided by the shear resistant skin layer. The skin stringers are connected to the shell skin in the vicinity of the feet of the stringers and likewise support the fibre strands. In addition, the shell skin and the stringers are mutually supportive whereby the stringers cater for the support of the shell skin in regard to buckling. Basically, unidirectional states of stress subsist in the stringers. Consequently, unidirectional stiffeners are preferably disposed in the stringer shells.
In order to provide an optimal weight for the construction and an optimal deformation behavior while retaining a high level of rigidity, the fibre stiffeners are globally aligned in accord with the direction of the cross-sectional parameters caused by the external load. The fibre strands are thereby oriented in the direction of the global frictional connections. If, for example, a slim structural element equipped in this manner is subjected to bending forces, there is a resultant, substantially mutually parallel alignment of the fibre strands over the width thereof. The state of stress in the fibre strands then remains uniaxial even when subjected to other loadings, for example, a transverse flection or shear deformation. Advantageously thereby, the fibre strands can be continuously dimensioned by making use of their fibre stiffness. Limits to their elongation need not be observed. The shell skin is merely designed in accord with the task appertaining to the specific application as regards the support of the fibre strands and the desired deformation behavior of the component. In regard to the partial cross-section thereof, the thickness of the skin can be continuously and independently varied.
The orientations and/or preferred directions of the material properties in the unidirectionally aligned fibre strands conform to the actual state of stress. The use of a carbon fibre reinforced synthetic material is particularly preferred. Due to the advantageous orientation of the fibers in the direction of the unidirectional stresses, the stiffness of the fibers and the strength thereof can be utilized to the full. Since the fibre strands and the shear resistant shell skin can, advantageously, be dimensioned independently of one another, it is possible to adapt them to the current requirements in a specific manner.
Fibre strands consisting of carbon fibers have proved to be advantageous due to their very high stiffs or very high strength in dependence on the actual type of fibre being used. Their specific characteristics are up to seven or eight times higher than those of an aluminum alloy for example, whereby the values in the case of a unidirectional loading are some 3.5-4 times higher than for a quasi-isotropic design. In the case of the latter, the stiffs are oriented equally in each direction, usually 0xc2x0, 90xc2x0 and xc2x145xc2x0. However, the directionally related performance of an anisotropic material is thereby lost whereas it is employed to advantage for the fibre strands in accordance with the invention. The fibers in the composite are loaded in each direction and thus subjected to transverse elongations i.e. stress components transverse to the fibers which lead to a premature failure of the structural element. Moreover, in comparison to metals, fibre materials have a substantially better fatigue behavior as long as they are only loaded in a direction along the fibers.
Preferably, there is provided a longitudinal supporting layer for accommodating longitudinal elongations and a shear supporting layer for accommodating shear deformations of up to predeterminable magnitudes. The orientations of the fibers in the laminate of the shell skin thereby amount preferably to xc2x145xc2x0 and/or 0xc2x0, whereby these angles may be modified somewhat from a constructional point of view. For the purposes of attaining a deformation coupling, the fibre strands must be oriented at an angle of xc2x1xcex2xe2x89xa00 (preferably 0xc2x0 to 25xc2x0) relative to the direction of the load. In order to support the coupling behavior, one or both orientations of the xc2x145xc2x0 layer may adopt an angle differing therefrom by up to about 10xc2x0. It is particularly preferred that the two layers be connected together and that a boundary layer be provided between the two layers for accommodating the forces arising from their combined deformation. It is advantageous therefore that 90xc2x0 layers should not be provided in the shell skin since these layers cannot arrest tears in the matrix and cause additional interlaminar loads. Through the use of fibre materials, immense differences in stiffness partially occur, for which reason there is a greater elastic extensibility when the 90xc2x0 layers are dispensed with.
It is preferred that the fibre strengtheners and/or stiffeners of the structural element be provided in the folds of the shell skin or the stringers. However, they may also be arranged in the region of the fibre bundles in the sheath. A further possibility consists in arranging them in the region of the sheer resistant laminate of the shell skin. Here, it is particularly preferred that they be provided in the form of a sewn reinforcement.
It is particularly preferred that the stringers of the shell skin be included with the alignment of the fibre orientation. The structural elements can thereby be made to have a very light construction which then has a high tolerance to damage and provides a directed concept for the deformation behavior under load. The stringers form the buckling support means for the thin walled shell skin.
Preferably, one or more parting layers may be introduced. These may be provided either in an upstanding fold of the skin or a stringer, or within the shear resistant sheath of a fibre strand. This has proved to be particularly advantageous for preventing the formation of tears. The provision of a parting layer between the shear resistant sheaths of the unidirectional fibre strands leads to an interruption in the force flows between the unidirectional fibre strands, whereby the transverse tensile elongations and shear elongations at the bordering faces of the fibre cross-sections will be interrupted. In the case of longitudinal tensile loads, the cross-section of the fibre strands are drawn together due to the transverse contraction, whereby the individual strands gape apart and all of the normal and shear frictional connections are neutralized. In the case of longitudinal compressive loads, the cross-sections of the strands expand correspondingly, whereby the strands are pressed together. Transverse compressive stresses, but not shear stresses, thereby ensue in the cross-sections since the inserted parting layer allows the bordering surfaces to slide upon one another in a manner similar to a pile of boards sliding upon one another.
A particular advantage of the invention thus lies in that the number of structural variables can be reduced to a comprehendible level. The individual structural elements can be continuously dimensioned and their stiffness and strength can be aligned in precise accord with the global force flows specific to the application. Large jumps in stiffness no longer occur, for which reason the interlaminar stresses can be reduced to their least possible value. In the case of unidirectional fibre strands, the individual fibers or a roving can thereby be matched as the smallest unit specifically to the application. In the shell skin, which, in particular, is a torsion shell, this is preferably the xc2x145xc2x0 pair of laminates which usually has a layer thickness of 0.25 mm.
A further advantage lies in that the directed force flows can be influenced, for the purposes of achieving an optimal accommodation of the load, such that the desired deformation behavior of the whole structural element is achieved at the same time. It is thereby possible to produce a structural element which, on the one hand, is very flexible and, on the other hand, is highly rigid. The accommodation of large longitudinal and shear forces occurs separately in two interconnected layers. Each layer thereby bears the load components allotted thereto, and a border layer between the two layers preferably bears the forces caused by the combined deformation of the two layers. It is preferred thereby that the longitudinal elongation be accommodated by the longitudinal supporting layer and the shear deformation by the shear supporting layer.
It has proved to be particularly advantageous that, for sections in the loaded skin fields of a structural element, the longitudinal force flows can be diverted around these regions of the section by utilization of the construction of the structural element in accordance with the invention, since, due to the measures in accordance with the invention, the result is achieved that the state of stress within the fibre strands is substantially unidirectional. Concentrations of stress, which can occur in the region of the sections in shell skins or structural elements subjected to a load, can be balanced out by dimensioning the cross-section of the fibre strand. It is due to this feature precisely that the longitudinal force flows can be diverted around the weakened regions of the structural element.
For the purposes of producing dimensioned amounts of fibre stiffness perpendicular to the skin plane of the shell skin in the case of large structural elements, an RTM process functioning at differential pressures (Differential Pressure Resin Transfer Moulding) is used. An autoclave thereby provides the counter pressure required for the manufacturing process. A dry, semi-finished layered product is positioned in a molding cavity of the autoclave by means of a one-sided stitching technique, fixed and moistened with resin at a controlled flow speed.